The components of a reflex pathway—afferents, inter-neurons, and efferent neurons—exist within the extrinsic innervation to the GI tract. These reflexes can be mediated entirely via the vagus nerve (termed a vagovagal reflex), which has both afferent and efferent fibers. The vagal afferents send sensory information to the CNS, where they synapse with an interneuron, which then drives activity in the efferent motor neuron. These extrinsic reflexes are very important in regulating GI function after ingestion of a meal. An example of an important vagovagal reflex is the gastric receptive relaxation reflex, in which distention of the stomach results in relaxation of the smooth muscle in the stomach; this allows filling of the stomach to occur without an increase in intraluminal pressure.
In general, as with other visceral organ systems, the parasympathetic and sympathetic nervous systems tend to work in opposition. However, this is not as simple as in the cardiovascular system, for example. Activation of the parasympathetic nervous system is important in the integrative response to a meal and is discussed in the following chapters. The parasympathetic nervous system generally results in activation of physiological processes in the gut wall, although there are notable exceptions. In contrast, the sympathetic nervous system tends to be inhibitory to GI function and is more frequently activated in pathophysiological circumstances. Overall, sympathetic activation inhibits smooth muscle function. The exception to this is the sympathetic innervation of GI sphincters, in which sympathetic activation tends to induce contraction of smooth muscle. Moreover, the sympathetic nervous system is notably important in regulation of blood flow in the GI tract.
The ENS is made up of two major plexuses, which are collections of nerve cell bodies (ganglia) and their fibers, all originating in the wall of the gut (
Fig. 27.8). The myenteric plexus lies between the longitudinal and circular muscle layers, and the submucosal plexus lies in the submucosa. Interganglionic strands link neurons in the two plexuses.
Neurons in the ENS are characterized functionally as afferent neurons, interneurons, or efferent neurons, similar to neurons in the extrinsic part of the ANS. Thus all components of a reflex pathway can be contained within the ENS. Stimuli in the wall of the gut are detected by afferent neurons, which activate interneurons and then efferent neurons to alter function. In this way the ENS can act autonomously from extrinsic innervation. However, neurons in the ENS, as we have already seen, are innervated by extrinsic neurons, and thus the function of these reflex pathways can be modulated by the extrinsic nervous system. Because the ENS is capable of performing its own integrative functions and complex reflex pathways, it is sometimes referred to as the “little brain in the gut” as a result of its importance and complexity. It is estimated that there are as many neurons in the ENS as in the spinal cord. In addition, many GI hormones also act as neurotransmitters in the ENS and in the brain in regions involved in autonomic outflow. These mediators and regulatory peptides are thus • Fig. 27.8 The enteric nervous system in the wall of the GI tract.
referred to as brain-gut peptides, and the extrinsic and intrinsic components innervating the gut are sometimes referred to as the brain-gut axis.
Response of the GI Tract to a Meal